Conventionally, there has been a screw compressor in which, as shown in an enlarged sectional view of FIG. 8, a screw rotor 102 is housed in a cylinder 110 of a casing 101 and a gate rotor 103 is engaged with the screw rotor 102 so that gas compression is fulfilled by a compression chamber defined by mutual engagement of the screw rotor 102 and the gate rotor 103 (see JP 3731399 B2).
That is, as shown in FIG. 9, which is taken along the line B-B of FIG. 8, groove portions 121 of the screw rotor 102 and tooth portions 131 of the gate rotor 103 are engaged with each other, respectively, to form the compression chamber. Then, a low-pressure gas is sucked into the compression chamber from one end side of the screw rotor 102 in its axis 102a direction. After the low-pressure gas is compressed in the compression chamber, the compressed high-pressure gas is discharged from the other end side of the screw rotor 102 in its axis 102a direction.
In FIG. 9, the left side of the screw rotor 102 as viewed in the drawing sheet is assumed as an inlet side on which the gas is sucked into the compression chamber, while the right side of the screw rotor 102 in the drawing sheet is assumed as an outlet side on which the gas is discharged from the compression chamber.
As shown in FIGS. 8 and 9, between one surface 130 of the gate rotor 103 and a seal surface 111 of the casing 101 opposed to the one surface 130 is a slight gap, by which contact of the seal surface 111 of the casing 101 and the one surface 130 of the gate rotor 103 with each other is prevented. A width W of the seal surface 111 is uniform over a range from inlet side to outlet side of the screw rotor 102.